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Relativity refers to two [[physics]] theories; general relativity (GR) and special relativity (SR), popularized by [[Albert Einstein]]. General relativity is a geometrical theory of gravitation, while special relativity is a limiting case. Einstein explains special relativity in terms of two postulates:

'''Relativity''' refers to two theories in [[physics]], and to a principle which led to the first theory. The theories, both developed by [[Albert Einstein]], are '''special relativity''' (SR) and '''general relativity (GR). Special relativity is a theory which describes the laws of motion for non-accelerating bodies traveling at a significant fraction of the speed of light; at speeds approaching zero, special relativity is identical to Newton's Laws of Motion. General relativity is a theory which explains the laws of motion as viewed from accelerating reference frames and includes a geometric explanation for gravity.

 

Einstein derives special relativity from two postulates:

# ''The [[speed of light]] is constant for all (inertial) observers, regardless of their velocities relative to each other.''

# ''The [[speed of light]] is constant for all (inertial) observers, regardless of their velocities relative to each other.''

# ''The laws of physics are obeyed in all reference frames.''

# ''The laws of physics are identical in all inertial reference frames.''

Relativity was first proposed based on mathematical theory developed by [[Henri Poincaré]] and [[Hendrik Lorentz]]. This theory differs from [[Isaac Newton]]'s theory of gravitation by disposing with the idea of a universal, mutually agreeable scale of time (i.e. a universal clock that all times can refer to) and space (i.e. a universal sheet of "graph paper", which location refers to).  At low speeds (relative to light-speed), the Einstein-Lorentzian relativity equations are equivalent to Newton's formulas.

Einstein's theory of Special Relativity was based on theory developed by [[Henri Poincaré]] and [[Hendrik Lorentz]], working on problems in electrodynamics and the [[Michelson-Morley experiment]], which had not found any sign of [[luminoferous ether]], which was believed to be the substance which carried electromagnetic waves. Special relativity alters [[Isaac Newton]]'s laws of motion by assuming that the speed of light will be the same for all observers, despite their relative velocities and the source of the light. (Therefore, if A sends a beam of light to B, and both measure the speed, it will be the same for both, no matter what the relative velocity of A and B. In Newtonian/Galiliean mechanics, If A sends a physical object at a particular velocity towards B, and nothing slows it, the velocity of the object relative to B depends on the velocities of the object and of B relative to A.)

The famous equation attributed to Einstein, ''E=mc²'', describes the relationship between energy and the rest mass of a body.

At low speeds (relative to light-speed), the Einstein-Lorentz relativity equations are equivalent to Newton's equations. The famous equation attributed to Einstein, ''E=mc²'', describes the relationship between energy and the rest mass of a body.

In general terms, relativity predicts that space-time can be curved by massive bodies, so that (for example) near a [[black hole]] the sum of the angles in a triangle is not exactly 180 degrees, time passes more rapidly away from a black hole than near it (for a distant observer) and other apparent violations of [[geometry]] and common sense.

General relativity predicts that space-time is curved by massive bodies, so that near any massive body, the sum of the angles in a triangle is not exactly 180 degrees.  

Relativity is important for massive or fast-moving bodies: at low mass and low speed, it can be accurately approximated by [[classical mechanics]] (such as [[Isaac Newton]]'s laws of motion). At the two extremes, modeling the behavior of electrons requires that relativistic effects be taken into account (the chemically significant phenomenon of [[electron spin]] arises from relativity), and the course light passing through a region containing many massive bodies such as galaxies will be distorted ([[classical mechanics]], in which light travels with infinite speed in straight lines, does not predict this). These are both experimentally confirmed ([[electron spin]] was known before relativity arose, and telescopic observations confirm that galactic clusters distort the paths of the light passing through them).  

Relativity is important for massive or fast-moving bodies: at low mass and low speed, it can be accurately approximated by [[classical mechanics]] (such as [[Isaac Newton]]'s laws of motion). At the two extremes, modeling the behavior of electrons requires that relativistic effects be taken into account (the chemically significant phenomenon of [[electron spin]] arises from relativity), and the course light passing through a region containing many massive bodies such as galaxies will be distorted ([[classical mechanics]], in which light travels with infinite speed in straight lines, does not predict this). These are both experimentally confirmed ([[electron spin]] was known before relativity arose, and telescopic observations confirm that galactic clusters distort the paths of the light passing through them).